Approach to the adult with knee pain likely of musculoskeletal origin

INTRODUCTION — As the knee has the largest articulating surface of any joint and is weightbearing, it is not surprising that it is among the most commonly injured body parts. Acute knee pain accounts for over one million emergency department visits and more than 1.9 million primary care outpatient visits annually in the United States alone [1-3].

The evaluation of knee pain most likely caused by musculoskeletal injury from acute trauma, chronic overuse, or a combination of these, particularly as this may occur in athletic and active adults, is reviewed here. The topic includes details about obtaining an effective history and a general scheme for differentiating among the causes of musculoskeletal knee pain based upon the history and key clinical findings. Discussions of how to approach undifferentiated knee pain in the adult and of specific causes of knee pain are found separately. (See "Approach to the adult with unspecified knee pain" and "Patellofemoral pain" and "Meniscal injury of the knee" and "Clinical manifestations and diagnosis of osteoarthritis" and "Knee bursitis" and "Anterior cruciate ligament injury" and "Running injuries of the lower extremities: Risk factors and prevention".)

BASIC KNEE ANATOMY AND BIOMECHANICS — The anatomy and basic biomechanics of the knee are reviewed separately. (See "Physical examination of the knee", section on 'Anatomy'.)

HISTORY

General approach — Taking an appropriate history remains essential to directing the work-up of the patient with knee pain. Classically, a complete history determines the initial differential diagnosis, which guides the subsequent evaluation, ultimately leading to the correct diagnosis and treatment. However, as the differential diagnosis for knee pain is complex, a more practical approach focuses on the patient’s history, key symptoms, and the most likely precipitating factor – trauma or overuse.

Based on these three features, patients can generally be placed in a likely "diagnostic category," or group of likely potential diagnoses. Based upon this diagnostic category, the clinician can determine which examination maneuvers to perform, and what laboratory tests and imaging studies may be needed to make the diagnosis.

Common diagnostic categories include:

●Acute knee pain following recent trauma or overuse

●Chronic knee pain associated with overuse

●Knee pain without trauma or overuse, possibly associated with systemic symptoms or signs

Other important features of the history include previous injury, past surgeries, medications (eg, quinolones), systemic disease, and the presence of any constitutional symptoms. (See 'History of prior knee injury or surgery' below and 'Systemic or constitutional symptoms' below.)

The accompanying tables review important historical characteristics, physical examination findings, and possible mechanisms for some major causes of knee pain in athletes and active adults (table 1 and table 2).

Definitions and key diagnostic distinctions

Acute injury from trauma or overuse — Acute trauma is typically easy to identify. Acute trauma is most simply defined as something abruptly going wrong. A collision between players, a skiing accident, and a fall from a height are common examples of acute trauma. But contact with another player or an object is not required. An athlete who experiences pain immediately after jumping, landing, cutting, squatting, slipping, or sprinting is classified as having experienced acute trauma. Pain from acute trauma typically stops the athlete from completing their intended activity, and the resulting injuries are often accompanied by inflammatory sequelae such as ecchymosis, swelling, or joint effusion.

Acute pain associated with overuse (or "overuse trauma") generally refers to pain that develops or increases abruptly after excessive activity. The pain associated with overuse trauma typically arises towards the end of an activity that exceeds what the athlete has trained to do. A classic example is the runner accustomed to running 3 miles (5 km) daily who decides to do a 15-mile (24 km) run. This individual might develop significant anterior knee pain and possibly an effusion from excessive stress on the patellofemoral joint complex. Overuse trauma from excessive activity is associated with a progressive pain pattern that causes increasing functional limitations or complete cessation of activity.

Acute versus chronic pain — Acute and chronic pain are classically distinguished by duration. For most musculoskeletal conditions, pain for less than six weeks is usually described as acute or subacute, while pain lasting longer than six weeks is typically characterized as chronic. While the six-week threshold is arbitrary, it can be useful since many self-limited injuries heal by the end of six weeks with appropriate rest.

Acute pain typically has no progressive pattern. In response to a particular trauma, acute pain flares, lasts a predictable period, and then subsides as healing occurs. Chronic pain has a more progressive pattern. As an example, chronic knee pain from iliotibial band syndrome may initially only be bothersome at the end of a long run, but may progress over weeks to the point where pain develops after five minutes of running, or is even present at rest.

Characterizing symptoms and the mechanism of injury — Detailed questions regarding exactly how, when, and where any injury occurred, and about when and where the knee pain began are essential. Below are a number of key questions that can help characterize knee pain and related symptoms.

Questions to help characterize pain:

●What were you doing immediately prior to and at the time pain began? How were your leg and body positioned?

The exact activity at the time of injury can help to determine the diagnosis. As an example, a basketball player whose knee buckles while landing after a jump shot, followed by rapid knee swelling, gives a history suggesting anterior cruciate ligament (ACL) injury. When patients experience significant pain or swelling without sport or work activity, a systemic condition triggering the knee symptoms is more likely.

●Did you have pain immediately at the time of injury or during exercise?

Immediate pain with an injury or during exercise likely represents "macro trauma" involving structural damage to the knee.

●If not immediate, how long after exercise does the pain begin?

Delayed pain or soreness suggests overuse-related "micro trauma" involving tendons with minor structural breakdown, cartilage contusions, minor tears of soft tissue, or muscle strains. The length of the delay may have clinical significance. As an example, pain after four hours likely represents a more severe injury than soreness after 24 hours.

●Is the pain intermittent or constant?

Constant pain suggests either major injury or an active inflammatory process. Intermittent pain suggests a relatively minor injury or possibly excessive activity in the case of an arthritic knee.

●Does the pain only occur with specific activities?

Specific activities that trigger pain can be extremely helpful in diagnosing knee pathology. As an example, pain with prolonged sitting or when climbing or descending stairs is a classic part of the history of patellofemoral pain (PFP).

●Has the pain prevented you from performing (physical activity of choice)? Have you been able to resume doing (physical activity)?

The answers to these questions help to determine how aggressively to intervene. When pain prevents sport or work activity, or even activities of daily living, more aggressive intervention is justified; if the patient has resumed activity, treatment can begin more gradually.

●If you have pain with weightbearing, does this pain resolve with nonweightbearing?

Answers to this question help to identify specific sports injuries. As an example, patients with a meniscal or articular cartilage injury may experience significant pain with weightbearing activity but minimal pain when swimming or biking. Iliotibial band (ITB) syndrome typically causes pain only with weightbearing activity involving knee flexion in a particular range; walking or running at a pace that does not involve this degree of flexion may not cause pain. PFP typically causes anterior knee pain only with running, walking down steps, or prolonged sitting with the knee bent. On the other hand, a severe injury causing an effusion is likely to cause constant pain. As examples, a torn collateral or cruciate ligament typically results in a constant aching pain that is not made especially worse by normal weightbearing.

●Is there pain at night?

Pain at night suggests significant structural damage; conditions like tendinopathy or PFP rarely cause night pain unless there is an associated tear. Pain at night is more common with bony injury, such as osteoarthritis or a significant intra-articular process like a stress fracture of the tibial plateau. Although much less common, bone tumors can present with pain that is worse at night.

●Does anything relieve the pain? Does anything exacerbate the pain?

Factors such as weightbearing or twisting, going from sitting to standing, running downhill or walking down stairs may be indicators of a likely mechanism of injury and probable diagnosis.

Questions to characterize other symptoms:

●Did you hear or feel a snap or pop associated with the injury?

If the patient describes hearing an audible pop, ACL or other ligament tear is a major concern. Teenagers and young adults who sustain an acute meniscal tear may also experience an audible pop at the time of injury.

●Following the injury, did the knee immediately begin to swell? Was any bruising or redness noted?

Rapid swelling following an acute injury occurs with bleeding into the knee joint, and occurs with significant tissue damage such as an ACL tear.

●Do you experience instability or a sensation of the knee "giving-way"?

It is important to distinguish true mechanical instability from pain-mediated instability. True instability occurs when the knee subluxates or "gives way" during a routine activity (eg, climbing stairs, walking) without pain preceding the episode. Such instability occurs with ligament tears and patellar instability. Pain-mediated instability occurs when knee or other lower extremity pain inhibits motor nerves controlling the quadriceps muscles causing the knee to buckle. This can occur with any number of painful conditions.

●Do you feel the knee is locking or getting stuck in place?

Locking of the knee suggests a mechanical block, as might occur with a meniscus tear or loose piece of cartilage. It is important to differentiate a true mechanical block from subjective symptoms of "grinding," "popping," or feeling that the knee is stuck or might pop if moved. Patients with a true mechanical block (which often requires surgical intervention) often say they can unlock the knee by massaging or moving it in a certain manner. Subjective symptoms are frequently caused by damage to the articular cartilage (for which surgery is generally not warranted, and may worsen symptoms) [4-6].

Correct understanding of the mechanism of a knee injury is central to proper diagnosis. Sideline physicians may have advantages when diagnosing acute traumatic knee pain if they witness the episode, but more commonly, the only "eyewitness" is the patient.

Injury mechanisms may be straightforward (eg, patient reports slipping on an icy field and slamming their knee straight into the ground). With athletes, however, determining the mechanism is often more difficult and can require an understanding of the sport, the player’s position, and the particular maneuver the athlete was performing. Additional details may be important, such as any protective equipment that was worn, field and weather conditions, and the actions of other players. As an example, few ACL tears occur on wet fields, where the shoe-surface interface is unlikely to allow athletes to generate sufficient stress to rupture the ligament [7].

In some cases, an injury may arise from a number of possible mechanisms. As an example, patellar subluxation may be caused by any of the following: acute trauma; chronic overuse leading to strength imbalances between the vastus lateralis and the vastus medialis obliquus; failed lateral release surgery or overly aggressive resection of a medial plica; and Ehlers Danlos or other types of hypermobility syndromes causing recurrent subluxation.

History of prior knee injury or surgery — The most predictive risk factor for future knee injury is a history of prior injury. Patients should be carefully queried regarding the timeline and treatment of any past lower extremity injuries. Often "new" injuries turn out to be complications of old injuries (eg, patellofemoral pain from changes in running gait caused by plantar fasciitis). Surgery is another important risk factor for injury. Surgical repairs can fail, leading to symptoms from recurrent injury. In addition, athletes who undergo surgery that successfully restores function inevitably experience some degree of deconditioning that predisposes to new overuse injuries. This risk is greatest in athletes who undergo suboptimal rehabilitation or attempt to return to activities too quickly.

Systemic or constitutional symptoms — Patients with musculoskeletal pain should be asked whether constitutional symptoms, such as fevers, chills, night sweats, fatigue, or rash, accompany the pain. The presence of such symptoms suggests a systemic illness, and further investigation for infectious, autoimmune, or neoplastic causes is likely necessary. The differential diagnosis of joint pain related to systemic disease is reviewed separately. (See "Diagnosis and differential diagnosis of rheumatoid arthritis", section on 'Differential diagnosis' and "Approach to the adult with unspecified knee pain".)

The diagnosis of infectious disease can be complex in the setting of musculoskeletal complaints as infections can be the primary cause of knee pain (eg, Lyme disease, gonorrhea) or can secondarily complicate a traumatic process (eg, septic patellar bursitis). Patients with constitutional symptoms and a negative initial work-up may need referral for specialty evaluation.

PHYSICAL EXAMINATION OF THE KNEE — The physical examination of any joint is classically divided into inspection, palpation, range of motion testing, strength and neurovascular testing, and special maneuvers to assess for specific diagnoses. Special tests are selected based upon the most likely diagnostic category, which is based in turn upon the history, including the mechanism of any injury and the chronicity of pain. Guidance about which examination techniques are most useful for diagnosing specific conditions is provided in the text below, the accompanying tables (table 1 and table 2), and a topic devoted to the knee examination. (See "Physical examination of the knee".)

Telemedicine examination of the knee is reviewed separately. (See "Physical examination of the knee", section on 'Telemedicine examination'.)

DIAGNOSTIC CATEGORIES AND OVERALL APPROACH — Using information from the history, key symptoms, and findings from the basic knee examination, the clinician can usually select one of three common diagnostic categories that best fits the patient. The three major categories are:

●Acute knee pain, either from trauma or associated with overuse (table 1)

●Chronic knee pain associated with overuse (table 2)

●Knee pain without trauma or overuse, possibly associated with systemic symptoms (see 'Knee pain not associated with trauma or overuse' below)

Once a major diagnostic category is identified, the next step is to determine the precise diagnosis. The following sections of this topic provide guidance, including the appropriate use of special examination tests, about how to work through the common and important diagnoses within each category. While inspection, range of motion, and basic strength and neurovascular testing are typically performed in every patient with knee pain, the clinician determines which specific structures warrant more thorough evaluation and which special knee examination maneuvers to perform based upon the most likely diagnostic category.

The first important distinction to make when establishing the diagnostic category is whether the pain is acute or chronic. A history of acute knee pain associated with trauma is usually clear-cut, although the precise mechanism may be difficult to establish. Such a history allows the clinician to focus on the knee-related structures most likely to have been injured. These include the collateral and anterior cruciate ligaments and the menisci. Testing the stability of these structures, identifying points of focal tenderness and the presence of an effusion, and performing special tests to confirm suspicions about the structures involved comprise the core of the initial evaluation of acute trauma-related knee pain. (See 'Acute knee pain associated with trauma' below.)

Chronic knee pain associated with overuse is the major diagnostic category to consider if pain has persisted for approximately six weeks or longer and there has been no sudden inciting trauma associated with the development of pain. This is the category of knee pain seen most often in primary care practices. Chronic knee pain associated with overuse is typically progressive, becoming more painful with increasingly less intense activity over time. Typically, the knee examination reveals no structural instability. Many such patients suffer from patellofemoral pain (although this is a diagnosis of exclusion) or another condition related to the patella or surrounding soft tissues (eg, pes anserinus, iliotibial band). In older patients, a degenerative meniscus or osteoarthritis must be considered as both of these conditions are common, particularly in patients greater than age 40. (See 'Chronic knee pain associated with overuse' below.)

Patients with knee pain that developed or increased abruptly after excessive activity but who clearly have not sustained trauma of any kind often suffer from acute-on-chronic pain related to overuse ("overuse trauma"). The pain associated with overuse trauma typically arises towards the end of an activity that exceeds what the athlete has trained to do. A classic example is the runner accustomed to running 3 miles (5 km) daily who decides to do a 15-mile (24 km) run. When asked, these patients often report a progressive pain pattern that preceded the acute increase in pain and had caused increasing functional limitations or complete cessation of activity. Although the history is somewhat different, the causes of overuse trauma are generally the same as the causes of chronic knee pain from overuse, and the initial evaluation is the same.

Chronic or acute knee pain in an adult without inciting trauma or a history of overuse is a "red flag" that a more extensive evaluation is required, particularly if the pain is associated with constitutional symptoms, such as fever. The cause of such pain is much less likely to be musculoskeletal. (See 'Knee pain not associated with trauma or overuse' below.)

ACUTE KNEE PAIN ASSOCIATED WITH TRAUMA

Approach and differential diagnosis — In the setting of pain immediately following acute trauma, whether the trauma involved a direct blow or not (ie, non-contact injury), assessing for an effusion is a critical step. How to detect an effusion is described separately. (See "Physical examination of the knee", section on 'Detection of an effusion'.)

When looking for an effusion, it is important to note that complete tears of the collateral ligaments are often associated with tears of the knee joint capsule. When this occurs, effusion may be minimal or absent since capsule tears allow the intra-articular fluid to drain through the tear into the calf musculature. An important possible finding in effusion associated with capsular rupture is a tight, ecchymotic, or significantly swollen calf, which is often misdiagnosed as suspected deep vein thrombosis (DVT).

The differential diagnosis for acute knee pain following trauma associated with an effusion includes the following injuries (table 1):

Common:

●Medial or lateral collateral ligament tear

●Anterior cruciate ligament tear

●Meniscus tear

●Patellar dislocation or significant subluxation

●Patellar tendon tear

●Intra-articular fracture

●Osteochondral injury or defect

Less common:

●Bone contusion

●Posterolateral corner injury

●Posterior cruciate ligament tear

●Quadriceps tendon tear

●Fibular head or neck fracture

●Patella fracture

●Knee (tibiofemoral) dislocation

Accordingly, if an effusion is present or suspected, these structures should be carefully examined, including assessment with the special tests mentioned below. Wherever possible, these structures or areas should be palpated for tenderness and structural defects, and tested for stability and function. Performance of the knee examination is described separately; a table summarizing the major diagnoses and their distinguishing features is provided (table 1) (see "Physical examination of the knee"). When the diagnosis of effusion is questionable, ultrasound is a valuable adjunct to examination and shows high sensitivity and specificity when compared with the physical examination or magnetic resonance imaging (MRI) [8,9]. (See "Musculoskeletal ultrasound of the knee".)

If the history and examination of the patient with an acutely traumatized knee with a significant effusion suggests a diagnosis that may require surgical intervention (eg, anterior cruciate ligament [ACL] tear, intra-articular fracture, osteochondral defect), diagnostic imaging (often with MRI) is typically performed to confirm the diagnosis and determine the extent of injury. The decision to obtain advanced imaging can usually be made after discussion with the consulting orthopedic surgeon.

If no effusion is present, but pain developed immediately following trauma, the structures listed above remain the most likely source of pain and should be carefully assessed. In addition, a good general knee examination should be performed, including assessment of the structures associated with overuse injury. These are described below. (See 'Chronic knee pain associated with overuse' below.)

The structures and regions described immediately below require special testing for all patients with acute knee pain following trauma.

Diagnosing common and important injuries

History of acute stress against medial or lateral knee

Collateral ligament injury — Collateral ligament injury is likely in the following settings:

●History of trauma involving twisting of the leg or a direct blow leading to varus or valgus displacement (direct blow to inside or outside of knee is an important mechanism in contact sports such as American football or rugby). Valgus displacement (knee forced inwards) injures the medial ligament; varus displacement (knee forced outwards) injures the lateral ligament.

●Significant tenderness directly over either one of the collateral ligaments

●Positive varus (lateral collateral ligament) or valgus (medial collateral ligament) stress test (movie 1) (see "Physical examination of the knee", section on 'Assessing joint stability')

Medial (or tibial) collateral ligament — The medial collateral ligament (MCL) has its origin on the posterior aspect of the medial femoral condyle and inserts broadly 4 to 5 cm distal to the joint line along the medial tibia. The MCL provides the primary restraint to valgus motion of the knee. Injury of the MCL can occur along with damage to other structures (eg, meniscus, anterior cruciate ligament) or in isolation. Patients with an injured MCL often complain of vague medial-sided knee pain, but the combination of a suggestive history, focal tenderness over the ligament, and a positive valgus stress test help to confirm the diagnosis. (See "Medial (tibial) collateral ligament injury of the knee".)

Lateral (or fibular) collateral ligament — The lateral collateral ligament (LCL) provides the primary restraint to varus motion of the knee. The LCL is less vulnerable to injury than the MCL, and thus LCL injury often involves greater force and is associated with injuries to other static knee stabilizers, such as the anterior or posterior cruciate ligaments, which should be carefully assessed [10]. The combination of a suggestive history, focal tenderness over the ligament, and a positive varus stress test help to confirm the diagnosis. The role of the LCL in resisting varus angulation increases with knee flexion (as the structures of the posterolateral corner and the ACL become lax). Therefore, LCL integrity should be tested at both full knee extension and 35 degrees of knee flexion. (See "Lateral collateral ligament injury and related posterolateral corner injuries of the knee".)

Both collateral ligaments are readily visualized with musculoskeletal ultrasound (MSK US) and can be tested dynamically to evaluate for instability. However, evaluation is difficult given the size and three-dimensional complexity of the collateral ligaments [11]. Accordingly, the diagnosis and management of collateral ligament injuries remains largely clinical, and MSK US is a less important adjunct to diagnosis and treatment.

History of injury sustained from knee twisting, pivoting, landing, or rapid change of direction

Anterior cruciate ligament injury — Most acute anterior cruciate ligament (ACL) injuries are not sustained from a direct blow to the knee, but typically involve a sudden change of direction (eg, athlete making a cut or pivoting) or landing from a jump. The patient may hear a "pop" at the time of injury and the knee may be painful. Often, the patient feels the knee is unstable or "slipping out of place." Sometimes, patients make a "fist sign" in which they demonstrate how the knee feels like it is slipping forward by placing one fist on top of the other and sliding them against one another. An athlete may attempt to return to the game but is generally unable to play effectively. Notable knee swelling usually develops within 48 hours. An ACL injury is likely in patients with a suggestive history and asymmetric tests of knee stability. The most useful tests to evaluate the integrity of the ACL are the Lachman (picture 1), pivot shift (figure 1), and anterior drawer (picture 2) tests. A description of how to perform these maneuvers and a full discussion of ACL injury are provided separately. (See "Anterior cruciate ligament injury", section on 'Physical examination'.)

In general, MSK US is less effective at evaluating internal structures such as the ACL. However, preliminary studies using ultrasound to quantify differences between loaded and unloaded anterior tibial translation report high sensitivity for detection of acute and subacute ACL injury. In the United States, MRI remains the standard modality for confirming the diagnosis of ACL injury (algorithm 1). (See "Anterior cruciate ligament injury", section on 'Diagnostic imaging'.)

Posterior cruciate ligament and posterolateral corner injury — Patients with an injury of the posterior cruciate ligament (PCL) or posterolateral corner are less likely to remember an acute traumatic event and may present with vague, subacute or chronic symptoms [12-14]. If an acute mechanism is recalled, it typically involves a direct force applied to the proximal anterior tibia (eg, falling against a step, motor vehicle collision, football helmet striking the knee). These historical features, combined with a positive posterior drawer test (figure 2) and a posterior sag sign (picture 3), suggest a PCL injury. (See "Physical examination of the knee", section on 'Tests for PCL injury and posterior stability'.)

Posterolateral corner injury is characterized by damage to structures that support the posterior lateral corner of the knee [15-17]. Individuals with this injury have greater instability and less success returning to sports than those with isolated PCL tears. Key structures in the posterolateral corner that are susceptible to injury include the posterior lateral meniscus, popliteus tendon, iliotibial tract, lateral collateral ligament, knee capsule, lateral gastrocnemius muscle and tendon, and hamstring (biceps femoris) tendons. In addition to their other functions, these structures provide stability to the posterior and lateral knee, and resist tibial external rotation. Features that suggest injury to this area include symptoms of instability or pain with pivoting or twisting of the knee, focal tenderness at the posterior lateral corner, and a positive dial test. The degree of joint laxity relates to the severity of injury and the number of structures involved. (See "Lateral collateral ligament injury and related posterolateral corner injuries of the knee".)

Isolated acute injuries to structures in the posterior lateral corner are uncommon, and if suspected a careful evaluation of other commonly injured structures (eg, PCL, ACL) should be performed. In cases of an isolated injury to the posterior lateral corner, patients generally express vague complaints of posterior knee pain or instability with twisting or pivoting movements. An effusion is generally not present. Significant varus laxity noted in full extension indicates a complete LCL tear as well as concomitant disruption of the PCL or ACL disruption.

Meniscus tear — A history of an acute, forceful injury involving twisting of the knee while the foot is planted, combined with clinical findings of an effusion and positive special tests suggests a more severe meniscal lesion extending into the vascular "red zone" at the peripheral portion of a meniscus. A less severe meniscal injury limited to the relatively avascular "white zone" is more likely in the setting of an overuse or lower energy mechanism and the absence of a knee effusion. Patients with a substantial meniscal tear complain of knee pain and swelling, and typically describe the sensation of the knee locking or catching or not moving properly. Squatting or pivoting exacerbates the pain. (See "Meniscal injury of the knee", section on 'Mechanism and presentation'.)

The fibrocartilage menisci can be evaluated with a number of tests designed to elicit pain or reproduce the catching sensation or joint locking associated with a torn meniscus. However, the examination of the menisci is less predictive and less reproducible than tests of the anterior cruciate or collateral ligaments. The reliability and predictive value of meniscal tests seem to vary widely depending on examiner experience and the location of the injury. The Thessaly test may have high sensitivity and specificity, but studies to date are limited. Thus, it is best to consider the history carefully and to use several techniques to examine the menisci when trying to determine the likelihood of a tear. (See "Meniscal injury of the knee", section on 'Physical examination'.)

Patellar dislocation or subluxation — Patellar dislocation or subluxation occurs when the patella displaces from the trochlear groove, most often in a lateral direction (figure 3). Patients describe an acute, typically non-contact, injury involving pivoting or a sudden change in direction (cutting) causing the "knee" (ie, patella) to slip out of place. Most often, movements that place the knee in a flexed and valgus position are involved. As most patellar dislocations reduce spontaneously when the knee is fully extended, some patients never see their kneecap out of place. For the patella to dislocate completely, one of the medial restraining structures must tear. (See "Recognition and initial management of patellar dislocations".)

In the setting of an acute traumatic injury associated with an effusion, careful observation may reveal lateral asymmetry of the effusion and an unwillingness of the patient to fully extend their leg. Palpation may reveal the patella displaced from the sulcus, or possibly tenderness of the lateral retinaculum or other surrounding soft tissues. When the effusion seems most prominent anteriorly and the patient has marked tenderness along the medial border of the patella, a positive patellar apprehension test suggests an acute patellar dislocation that has spontaneously reduced, or a significant subluxation. In the setting of an overuse injury pattern, a positive patellar apprehension test suggests chronic subluxation or patellar instability, and little or no effusion may be apparent. Rare cases of medial subluxation or dislocation are seen primarily in patients with hypermobility syndromes or who have flattening and insufficiency of the retropatellar sulcus. In the setting of direct trauma to the patella, soft tissue swelling anterior to the patella may be related to a patellar fracture. (See "Patella fractures".)

History of sudden, forceful knee flexion

Patellar tendon tear — Complete tear, or rupture, of the patellar tendon is rare in active adults and occurs most commonly in patients under age 40 [18-20]. The injury typically occurs when a patient’s knee is flexed 60 degrees or more and sustains a force that suddenly increases the degree of flexion. A classic example of this injury occurs during volleyball when an athlete descending from a spike lands on another player’s foot in such a manner that the already flexed knee suddenly has to flex more acutely while absorbing the force from of the landing. In such circumstances, the patellar tendon avulses from the distal patellar pole. If the rupture occurs within the substance of the tendon, the clinician should consider secondary causes, such as anabolic steroid abuse or recent treatment with quinolone antibiotics. The patient with a patellar tendon rupture often has significant knee swelling and pain, and cannot fully extend the knee. Examination usually reveals infrapatellar swelling and ecchymosis, tenderness at the inferior patellar border, and painful knee extension (partial tear) or inability to extend the knee (complete tear). A defect in the tendon may be palpable. Although uncommon, ACL tears or other injuries may be associated with patellar tendon rupture, and thus the knee should be carefully examined. (See "Quadriceps muscle and tendon injuries", section on 'Quadriceps and patellar tendon tears'.)

Rarely, patients experience bilateral patellar tendon ruptures. These usually occur in patients with a chronic soft tissue disease that predisposes to rupture or those taking glucocorticoids or quinolone antibiotics. A minority of cases have been described after a fall from a height or another traumatic mechanism [21].

If the patient experiences what they deem a serious knee injury that immediately limits their activity but examination reveals no impressive findings, the physician should assess for a partial tendon tear. Partial patellar tendon ruptures are relatively common particularly in athletes who perform explosive activity like jumping or sprinting [20]. Ultrasound evaluation often helps to diagnose partial tears in patients with acute symptoms but may also reveal changes consistent with chronic tendinopathy in athletes who have not reduced their activity, thus predisposing themselves to tendon injury. Patients with partial tendon tears can usually extend their knee almost completely but may experience pain or difficulty doing so. Knee swelling is typically less than is seen with a complete tendon rupture.

Quadriceps tendon tear — Although less common than the injuries described above (including patella tendon tear), adults with intense anterior knee pain and an acute hemarthrosis may have experienced a partial or complete tear of the quadriceps tendon [22-25]. Among athletes, participants in American Football sustain these injuries most often. Among adults generally, quadriceps tendon tears generally occur in those who are 40 years or older, and may be spontaneous and bilateral. Risk factors include obesity, diabetes mellitus, chronic kidney injury (ie, chronic renal failure), gout, hyperthyroidism, and chronic calcific tendinopathy. Tears have also been reported following glucocorticoid injection into the tendon. (See "Quadriceps muscle and tendon injuries", section on 'Quadriceps and patellar tendon tears'.)

The typical mechanism involves a fall backward while the foot is fixed and the knee is in some degree of flexion. A sudden fall in this position can create a strong eccentric contraction of the quadriceps tendon leading to a tear or avulsion from the patella. In American Football, a ball carrier who is tackled while in such a position is susceptible to this injury. Tears usually occur at the junction of the tendon and the superior patella. Typically the pain is severe enough that the patient will not bear weight and they cannot extend their leg. Additional findings include tenderness along the superior patellar border, significant swelling in the suprapatellar pouch, bruising around the site of injury, and an intact patellar tendon.

In skilled hands, MSK US can help to distinguish between high-grade and low-grade patellar and quadriceps tendon tears. While formal studies are lacking, numerous case reports describe accurate diagnosis of these injuries with MSK US [26,27].

History of direct trauma or forceful collision

Knee (tibiofemoral) dislocation — Complete dislocation of the knee (tibiofemoral joint) is a high-risk injury involving multiple knee ligaments that often compromises the vascular supply to the lower leg. Major trauma during sport and motor vehicle collisions account for a high percentage of cases. The classic mechanism involves a direct blow to the anterior knee region that produces 30 degrees or more of hyperextension. Dislocation can also occur in obese individuals who fall. Many knee dislocations are clinically obvious, but a substantial number reduce prior to medical evaluation, in which case the diagnosis can be difficult. A characteristic history and gross instability of the knee on examination suggest the diagnosis. A careful neurovascular examination is essential. (See "Knee (tibiofemoral) dislocation and reduction".)

Articular cartilage (osteochondral) injury or defect — Damage to the articular cartilage of the knee can occur from overuse (eg, osteochondritis dissecans) or from acute direct trauma [28,29]. Traumatic injuries to cartilage that cause significant effusions usually indicate a larger osteochondral defect (OCD) while lesser effusions occur with smaller OCD lesions or cartilage contusions. The area at greatest risk for such injuries is the medial femoral condyle. However, articular cartilage defects can occur on any condylar surface of the femur or tibia, especially weight-bearing surfaces. Athletes can sustain large articular cartilage defects during vigorous activity that involves cutting and jumping. These lesions cause immediate pain and large effusions that can accumulate over a matter of a few hours, which typically prevents any immediate return to activity.

Although the history may be similar, examination findings with this injury differ from an ACL injury in that the knee typically does not demonstrate instability with a Lachman or anterior drawer test. Tenderness may be present over the lateral portion of the medial femoral condyle (most common site of injury) with the knee flexed. However, studies of military personnel with an ACL injury have found associated osteochondral defects in nearly half of patients [30]. Thus, the clinician may have difficulty defining the extent of injury without an MRI study or arthroscopy. Substantial cartilage defects often require surgery and can shorten athletic careers. (See "Anterior cruciate ligament injury", section on 'Physical examination'.)

Intra-articular fractures — Intra-articular fractures can cause an acute knee effusion and possibly discrete bony tenderness in the region of the fracture. Most fractures involve the tibial plateau, with 60 percent occurring on the thinner lateral plateau [31]. High velocity injuries are more common in patients younger than 40, with 80 percent due to motor vehicle collisions. Athletes can sustain such fractures during sport, most often from a significant valgus stress on the knee during a fall or an awkward landing. A small percentage of the injuries sustained in sports are stress fractures. Older adults, particularly females with osteoporosis, can sustain such fractures from low velocity trauma such as an awkward step or a fall from standing. (See "Proximal tibial fractures in adults" and "Overview of stress fractures" and "Overview of the management of low bone mass and osteoporosis in postmenopausal women".)

The diagnosis is more likely if aspiration of the knee reveals a bloody effusion. However, there are no definitive physical examination maneuvers to detect an intra-articular fracture. In the setting of an appropriate history, the combination of stable ligaments and rapid development of a knee effusion suggests the diagnosis. If ligamentous instability and significant effusion are both present, the injury may include a ligament tear and an intra-articular fracture [32]. In either case, diagnostic imaging is necessary to determine the diagnosis. Although plain radiographs may reveal an intra-articular fracture, generally MRI or computed tomography (CT) is needed to delineate the full extent of the injury.

Fibular head or neck fracture — A direct blow to the proximal fibula resulting in a bony contusion or fracture can cause lateral knee pain that presents with or without swelling. Severe medial sided ankle sprains or medial malleolar fractures can also cause a fibular neck or head fracture (Maisonneuve fracture (image 1)). Therefore, the lateral knee should be examined following any severe medial ankle injury. Any of the injuries described produce pain with joint motion and focal bony tenderness at fibular head or neck; palpation of the joint line and other structures not connected to the proximal fibula should not produce tenderness.

Patella fracture — Direct trauma to the anterior knee can cause a fracture of the patella. Pain, swelling, and ecchymosis are typically localized to the anterior knee directly over the patella, unless the fracture is accompanied by other injuries. Due to the mechanism, posterior cruciate ligament tears can accompany patella fractures so knee joint laxity should be assessed, as should the integrity of the extensor mechanism assessed. Radiographs generally reveal the fracture, but special views may be needed. (See "Patella fractures".)

Bone contusion — Bone contusions occur during acute injuries and can present with focal or diffuse knee pain, which can be severe. They can be intra-articular or extra-articular, and may or may not cause a joint effusion. Bone contusions are subchondral lesions identified by MRI. Particular types of bone contusions are commonly associated with specific non-bony injuries. As an example, a large percentage of ACL injuries are associated with contusions involving the posterior lateral tibial plateau and the anterior lateral femoral condyle (at the sulcus terminalis) [33]. A bony contusion at the anterior aspect of the lateral femoral condyle or the medial patellar facet raises the possibility of patellar subluxation.

Imaging in the evaluation of acute knee pain — Diagnostic imaging is used as an adjunct to the history and physical examination when evaluating the adult with knee pain. The suspected clinical diagnoses determine the need for imaging and appropriate study. (See "Radiologic evaluation of the acutely painful knee in adults".)

Standard radiographs — Following acute trauma, imaging typically begins with plain radiographs (image 2). Validated clinical prediction rules such as the Ottawa Knee Rule and the Pittsburgh Knee Rule help the clinician to determine when plain radiographs should be obtained after acute injury:

Ottawa Knee Rule (OKR) — The OKR states that radiographs of the knee should be obtained after acute injury ONLY for patients who meet one or more of the following criteria [34]:

●Age ≥55 years

●Isolated tenderness of patella (with no other bony tenderness of the knee)

●Tenderness at the head of the fibula

●Inability to flex the knee to 90 degrees

●Inability to bear weight both immediately and in the emergency department for four steps, regardless of limp (ie, unable to transfer weight onto each lower limb two times)

In a meta-analysis of eight studies involving 7385 patients, the OKR demonstrated a sensitivity just under 99 percent for identifying patients with knee fractures (95% CI 0.97- 1.00) and a specificity of 49 percent (95% CI 0.47-0.51) [35].

Pittsburgh Knee Rule (PKR) — The PKR criteria include [36]: Blunt trauma or a fall as mechanism of injury plus either of the following:

●Age younger than 12 years or older than 50 years

●Inability to walk four weight-bearing steps immediately after or in the emergency department, regardless of age

In a head to head study the PKR was just as sensitive and significantly more specific than the OKR in adults with an isolated knee injury [36].

Ultrasound — In skilled hands, ultrasound provides a useful point-of-care technique for confirming a joint effusion and diagnosing soft tissue injuries around the knee, including injuries of the adjacent tendons and the collateral and cruciate ligaments. Performance of the knee ultrasound examination is described separately; test characteristics of ultrasound are reviewed in the topics devoted to particular conditions. (See "Musculoskeletal ultrasound of the knee".)

MRI and CT with or without arthrography — MRI and CT are most useful for suspected intra-articular problems. In most medical centers, MRI is the standard technique used to diagnose ACL, PCL, and meniscal injuries. Other advanced imaging is used selectively for diagnosing chondral lesions, intra-articular fractures, and diagnostic challenges [37]. (See "Approach to the adult with unspecified knee pain".)

CHRONIC KNEE PAIN ASSOCIATED WITH OVERUSE

Approach and differential diagnosis — Patients with chronic knee pain range from elite athletes involved in extreme conditioning programs to beginning joggers. The differential diagnosis for chronic knee pain, including a description of distinguishing features, is provided in the accompanying tables (table 2 and table 3).

For the purposes of this topic, chronic knee pain associated with overuse is pain occurring during or after specific activities that has persisted for approximately six weeks or longer. Chronic knee pain occurring without physical activity is considered separately. (See 'Knee pain not associated with trauma or overuse' below.)

Chronic knee pain associated with overuse is typically progressive, becoming more painful with increasingly less intense activity over time. In patients whose history fits this description, examination of the following structures is essential:

●Patellofemoral joint

●Iliotibial band

●Pes anserinus tendon and bursa

●Patellar and quadriceps tendons

●Tibial tuberosity (for adolescents and young adults)

●Lateral and medial meniscus

●Knee joint (for osteoarthritis)

In addition, assessment of the tightness of the hamstring, quadriceps, and hip flexor muscles can be helpful.

Note that certain chronic knee conditions can occasionally cause acute pain and even an effusion. One common example is the patient with known degenerative osteoarthritis of the knee. Such patients can develop acute pain and even a mild effusion and warmth over the knee with relatively mild overuse. Another example is the patient with chronic patellofemoral pain who may acutely develop an effusion after excessive activity.

Muscle inflexibility — Tight musculature is not a primary diagnosis and alone does not explain a patient’s knee pain. However, restricted hamstring flexibility is a common secondary finding in patients with patellofemoral pain and pes anserinus pain, while quadriceps or hip flexor tightness serves as secondary findings in patients suffering from patellofemoral pain, patellar tendinopathy, Osgood Schlatter syndrome, and other types of chronic knee pain. Thus, the secondary finding of decreased muscle flexibility can help to confirm the primary diagnosis and suggest one possible treatment.

Diagnosing common conditions

Conditions causing anterior or medial knee pain

Assessment of the patella and surrounding soft tissue — In patients with knee pain from overuse, proper assessment of the patella and extensor mechanism is often the key to diagnosis. Over 70 percent of visits for knee pain to outpatient physicians are for patellofemoral pain [1]. Careful palpation of the patella and its associated structures can assist with diagnosis. (See "Patellofemoral pain".)

Important structures to palpate include the following:

●Patella and patellofemoral joint

●Patellar tendon, including insertion at tibial tubercle

●Quadriceps muscle and tendon

Focal tenderness of the patella may indicate a fracture. Patellar fractures usually present with acute pain following acute trauma, but chronic pain from a nonhealing fracture is possible. Tenderness to direct patellar palpation should be investigated initially with plain radiographs. (See "Patella fractures".)

Swelling of the soft tissue over the patella is sometimes mistaken for an effusion. However, in the setting of overuse trauma or chronic pain, swelling immediately over the patella is typically due to prepatellar bursitis. The infrapatellar bursa more commonly causes swelling deep to the patellar tendon and closer to the tendinous insertion of the tibial tubercle. With either type of bursitis, the knee joint proper (ie, tibiofemoral joint) is uninvolved and thus knee motion remains unaffected and no pain should be felt deep within the joint during flexion and extension. A swollen, boggy bursa should be palpable. (See "Knee bursitis".)

Patellofemoral pain — Patellofemoral pain (PFP) can be defined as anterior knee pain involving the patella and retinaculum that excludes other intra-articular and peripatellar pathology. PFP appears to be multifactorial, resulting from a complex interaction among intrinsic anatomic and extrinsic training factors. It occurs commonly in runners. The causes, diagnosis (including examination maneuvers), and management of PFP are discussed in detail separately. Other conditions can also affect the patellofemoral joint (eg, osteoarthritis) and result in similar findings on patellofemoral testing. (See "Patellofemoral pain".)

Patients with PFP generally complain of anterior, peri-patellar knee pain that increases with squatting, prolonged sitting, climbing or descending stairs, or running (particularly downhill). Examination techniques for detecting patellofemoral irritation include palpation for retropatellar tenderness, the patellar compression test, and the patellar inhibition (or Clarke’s) test. The combination of positive findings on one or several patellofemoral tests, a history of overuse (most often involving running), including an insidious onset of pain, and the absence of an effusion and constitutional symptoms suggests the diagnosis of PFP. A weak vastus medialis oblique and gluteus medius (hip abductor) are common. Similar findings in an older athlete complaining of a classic diurnal pain pattern are consistent with patellofemoral osteoarthritis.

Patellar tendinopathy — Focal tenderness of the patellar tendon usually indicates patellar tendinopathy in the skeletally mature athlete. Patellar tendinopathy may be associated with excessive tightness of the quadriceps and/or hip flexor muscles, and causes pain with jumping, running, and other ballistic activities. In addition to tenderness at the inferior pole of the patella, palpation typically reveals a thickened tendon compared to the unaffected knee. Knee motion is unaffected, but squatting or hopping will reproduce the pain. Ultrasound can demonstrate patellar tendinopathy and show the degree of pathologic change, as well as identify partial tears of the tendon. (See "Patellar tendinopathy".)

Chronic patellar subluxation — Patients with a history of patellar dislocation may later develop chronic subluxation from damage to structures that constrain medial movement of the patella. Other individuals with shallow patellofemoral grooves may be prone to chronic subluxation if they have vastus medialis weakness. Individuals with Ehlers Danlos or other hypermobility syndromes are also susceptible to chronic subluxation. All such patients typically complain of medial peripatellar pain and often describe the knee as giving way. The patellar apprehension test is usually positive, and often the vastus medialis is atrophic and weak. (See "Patellofemoral pain", section on 'Special tests'.)

Pes anserinus (medial hamstring) tendon and bursa — The pes anserine tendon is the confluence of the sartorius, gracilis, and semitendinosus (medial hamstring muscles) tendons where they insert on the tibia at the medial-anterior aspect of the knee (figure 4 and picture 4). Tendinopathy of these tendons or irritation of the bursa that lies between the tendons and the medial tibial condyle causes an insidious pain that ranges in severity from nagging to completely debilitating. In older women, pes anserinus tendinopathy is associated with genu valgus ("knock knees") and medial knee instability [38]. Findings include puffiness or swelling at the proximal medial-anterior tibia not seen on the opposite leg. Palpation of the three tendons starting just posterior and medial to the knee down to their insertion may reveal tenderness. Pain may be elicited by resisted knee flexion (performed on a prone patient with the hip adducted and internally rotated) or resisted hip adduction (performed with the patient on their side with their knee extended and hip internally rotated). (See "Knee bursitis".)

Osteoarthritis flare — Osteoarthritis involves degradation and thinning of the articular cartilage. When subjected to overuse trauma, the degraded cartilage is further damaged, resulting in an increase in knee pain and a small to moderate effusion. Patients with an osteoarthritis flare often describe a delayed onset to their effusion, which develops 12 to 24 hours following the acute event. Vague or diffuse joint line tenderness, intact ligaments, and non-focal meniscus tests comprise the typical constellation of examination findings associated with osteoarthritis flares. In addition, many patients are unable to fully flex or extend the affected knee (picture 5). Standing knee films best demonstrate the degree of cartilage loss and are the initial diagnostic test in OA (image 3 and image 4 and image 5). Osteochondral deficits may be seen on plain radiograph, but magnetic resonance imaging (MRI) is required for full characterization and a precise diagnosis. (See "Clinical manifestations and diagnosis of osteoarthritis".)

Many older athletes choose to continue participating in sports despite significant OA of the knees. For these individuals, treatments to minimize symptomatic flares including over-the-counter medications and even periodic injections (eg, glucocorticoid) may allow them to continue their activities but will not diminish the impact that weightbearing sport has on knee OA. Compression sleeves and knee braces may lessen symptoms in some older athletes, but have not been shown to slow the progression of OA.

Tibial tuberosity — Tenderness of the tibial tuberosity in an older child or adolescent strongly suggests Osgood Schlatter disease (apophysitis of the proximal tibial tubercle at the insertion of the patellar tendon (picture 6 and image 6)). Associated findings include tightness of the quadriceps and hip flexor muscles, pain that increases with activity and improves with rest, and the absence of any constitutional symptoms (see "Osgood-Schlatter disease (tibial tuberosity avulsion)").

Sinding Larsen Johansson syndrome is an apophysitis affecting the distal patellar apophysis that causes similar symptoms in the same age group but occurs less often. Assessment of the child or adolescent with knee pain is discussed separately. (See "Approach to chronic knee pain or injury in children or skeletally immature adolescents" and "Approach to acute knee pain and injury in children and skeletally immature adolescents".)

Quadriceps tendinopathy — Focal tenderness of the quadriceps tendon in the area between the superior border of the patella and the body of the quadriceps muscles (picture 7 and figure 5) suggests either quadriceps tendinopathy in the setting of chronic pain or a quadriceps tendon tear in the setting of acute pain following trauma [39]. Both conditions are uncommon. Quadriceps tendinopathy may be associated with excessive tightness of the quadriceps and/or hip flexor muscles, and causes pain with jumping, running, squatting, and other ballistic activities. Long-term involvement in running or sports involving jumping results in chronic changes to the tendon, including partial tears and calcification or traction spurs – small calcific bone spurs emanating from the superior border of the patella that extend into the quadriceps tendon. These changes are readily seen with ultrasound. Examination findings include pain with resisted knee extension and often some atrophy of the quadriceps muscle (most often the vastus medialis) on the involved side in comparison with the unaffected leg. (See "Quadriceps muscle and tendon injuries", section on 'Quadriceps tendinopathy'.)

Medial plica syndrome — Individuals who have sustained trauma to the medial peripatellar area or dislocations or subluxations of the patella may develop thickening of the medial patellar plica (figure 6 and image 7) [40]. This condition can also develop from overuse, particularly in runners with some degree of genu valgus ("knock knees"). A thickened medial plica causes impingement of the medial edge of the patella, and sometimes chondral injury, leading to localized pain that increases with movement. Examination reveals thickening of the plica with focal tenderness at the medial underside of the patella. Abnormal tracking of the patella during flexion and extension, sometimes accompanied by an audible pop emanating from the area of the medial patella, along with the other findings described, suggest the diagnosis. (See "Plica syndrome of the knee".)

Conditions causing lateral and posterior knee pain

Iliotibial band — The iliotibial band (ITB) runs from the ilium to the tibia, and may cause knee pain where it crosses the lateral femoral condyle (figure 7 and figure 8). The diagnosis of ITB syndrome is suggested by a history of insidious lateral knee pain that worsens with prolonged exercise and tenderness of the ITB in the region of the lateral femoral condyle. ITB syndrome is most common in runners but also occurs in cyclists and other athletes. ITB tightness is classically associated with ITB syndrome and is assessed with the Noble and other tests. Hip abduction weakness is another common finding. ITB syndrome is discussed in greater detail separately. (See "Iliotibial band syndrome".)

In athletes with symptoms suggesting ITB syndrome, the diagnosis of popliteus tendinopathy should be considered. The popliteus tendon wraps around the posterior lateral corner of the knee and attaches to the lateral femoral condyle just below the femoral insertion of the LCL. In downhill running, the popliteus muscle and tendon prevent the lateral femur from translating forward on the tibia, and thus the tendon may be injured with excessive downhill running or speed work [41]. The condition is diagnosed based upon a suggestive history, most often involving posterolateral knee pain that increases when the athlete attempts to prevent acceleration ("braking") while running downhill, and tenderness along the posterior aspect of the lateral femoral condyle, best appreciated with the patient seated in a figure-of-four position (ie, hips flexed, abducted, and externally rotated, or FABER). Resisted external rotation of the tibia may also elicit pain. Ultrasound examination reveals tendinopathy and possibly a halo of hypoechoic fluid around the popliteus tendon. (See "Calf injuries not involving the Achilles tendon", section on 'Popliteus tendinopathy'.)

Other considerations in patients with symptoms suggesting ITB syndrome include a degenerative lateral meniscus (described below) or biceps femoris tendinopathy (involving lateral hamstring). Biceps femoris tendinopathy is suggested by pain around the tendon insertion with resisted knee flexion (performed with foot externally rotated), focal tenderness at this site, and characteristic findings on ultrasound. (See "Meniscal injury of the knee".)

Semimembranosus-gastrocnemius bursitis — Overuse of the semimembranosus and semitendinosus (medial hamstring) muscles may lead to a relatively uncommon bursitis along the medial-posterior aspect of the knee, where the tendons of these muscles cross over the tendon of the medial head of the gastrocnemius muscle. This bursitis can cause marked swelling that is often mistaken for an intra-articular effusion or a sign of deep vein thrombosis. When this degree of swelling occurs, significant tightness and pain develops over the medial, medial-posterior, and/or posterior border of the knee. Examination shows significant swelling, varying degrees of direct tenderness along the medial knee and distal medial hamstring tendons, and medial calf swelling or tightness. The involved bursa has a large potential space and on ultrasound or MRI has a characteristic U shape where it wraps around the tendons [42,43].

Degenerative or "white zone" meniscus tear — Acute traumatic injuries of a meniscus typically extend into the more peripheral "red zone," or vascular portion, resulting in pain and a relatively large effusion. Overuse trauma on the other hand, tends to damage only the inner "white" portion of the meniscus, which is avascular. Pain and effusions are typically less severe. With degenerative meniscus tears, the special tests used to detect injury (eg, Thessaly test (movie 2)) are usually positive. MRI studies show that a large percentage of adults over the age of 40 have degenerative meniscal tears, though many are asymptomatic. Thus, incidental tears identified on imaging studies may not account for a patient’s symptoms and must be interpreted in clinical context. (See "Meniscal injury of the knee", section on 'Physical examination'.)

Popliteal (Baker’s) cyst — Baker’s cysts arise from a damaged posterior knee capsule, which may be due to a hyperextension injury, osteoarthritis, direct trauma, or congenital causes. Cysts may cause posterior knee pain or remain asymptomatic. More significant findings develop if they cause compression of the neurovascular structures in the popliteal fossa [44]. The typical physical finding is of a palpable cystic structure in the popliteal fossa (picture 8 and picture 9). Popliteal cysts are readily diagnosed and can be effectively treated with the assistance of musculoskeletal ultrasound (MSK US) (image 8 and image 9 and image 10) [45]. (See "Popliteal (Baker's) cyst".)

ACUTE KNEE PAIN ASSOCIATED WITH OVERUSE — Most of the conditions described above that cause chronic knee pain from overuse can sometimes also cause acute flares of severe knee pain. Conditions commonly associated with such flares are listed in the following table (table 4). These flares can be confusing as it is sometimes difficult to determine whether the sudden increase in pain was caused by some minor trauma or represents an acceleration of a preexisting condition related to overuse, such as a tendinopathy. Often the history can help the clinician distinguish between these two possibilities. Key questions to ask include:

●Have you had any knee-related pain or other symptoms that preceded the acute episode of knee pain?

●What did you do differently in your training or sports activity at or around the time your knee pain developed?

●Did you change any specific equipment that you use for sport – shoes, bike fit, orthotics, exercise equipment, etc?

Typically, the patient whose acute knee pain stems from overuse has performed some activity (eg, a much longer run than usual) or made some change (eg, added hill running, changed running shoes) that caused the chronic issue to worsen acutely. The questions above can help prompt the patient to offer information that they may not have considered important.

Of the overuse conditions that flare acutely, patellofemoral pain (PFP) is probably the most common. More severe anterior knee pain is the key symptom, while peripatellar swelling and significant tenderness to patellar compression are key examination findings. Acute flares related to iliotibial band syndrome cause similar symptoms, but the pain is centered over the lateral femoral condyle. Examination may reveal mild swelling of the lateral knee and tenderness with provocative testing. (See "Patellofemoral pain" and "Iliotibial band syndrome" and "Running injuries of the lower extremities: Risk factors and prevention".)

Inflammation and swelling of the prepatellar, infrapatellar, or pes anserine bursa can occur acutely. Bursitis flares can be identified by tenderness, warmth, and swelling over the bursa in question, while the movement and stability of the knee joint itself remain unaffected. (See "Knee bursitis".)

Two other conditions that present with acute flares in older athletes are osteoarthritis and degenerative meniscus injury. Such flares cause a significant effusion, tightness around the knee, and difficulty performing full flexion or extension of the knee. (See "Clinical manifestations and diagnosis of osteoarthritis" and "Meniscal injury of the knee".)

Ligamentous instability is not a feature of any overuse injury. A physical examination showing instability in any plane suggests that the acute pain stems from a ligament or tendon injury secondary to trauma, even if the patient cannot recall or has difficulty describing the traumatic event.

KNEE PAIN NOT ASSOCIATED WITH TRAUMA OR OVERUSE — Knee pain in an adult without inciting trauma or overuse is a "red flag" that more extensive evaluation is required. A more detailed description of the general approach to knee pain in the adult is provided separately. (See "Approach to the adult with unspecified knee pain", section on 'Conditions NOT related to activity'.)

The presence of systemic symptoms, such as fevers, night sweats, chills, malaise, weight loss, or fatigue, strongly suggests that the problem is not simply musculoskeletal. In patients with knee pain unrelated to trauma or overuse but associated with systemic symptoms, a thorough physical examination should be performed, in addition to careful assessment of the knee. Depending upon the history and examination findings, additional testing including synovial fluid analysis, diagnostic imaging (usually MRI), and appropriate laboratory studies is likely to be necessary.

Determining whether an effusion is present is central to the evaluation of the patient with an acutely painful knee unrelated to trauma or overuse. Warmth over the knee, erythema, and signs of a joint effusion (not extra-articular swelling, as with prepatellar bursitis) mandate joint aspiration to evaluate for infection or inflammatory disease. If the joint fluid examination is benign and the patient is not systemically ill, advanced imaging may be pursued on a routine timeline and, if necessary, laboratory analysis may consist of a few simple screens for systemic infection. However, systemic symptoms or concerning synovial fluid results (eg, elevated white blood cell count) should prompt urgent evaluation. (See "Approach to the adult with unspecified knee pain" and "Joint aspiration or injection in adults: Technique and indications" and "Synovial fluid analysis".)

A history of repeated flares of bilateral knee pain that are increasingly severe suggests a rheumatologic or autoimmune condition. Known degenerative osteoarthritis of the knee is one common disorder that leads to knee pain and swelling. Such patients often develop a mild effusion and warmth over the knee without recognizing any change in activity. Flares in such cases are most often triggered by relative overuse, and these patients can usually be treated symptomatically. Systemic lupus erythematosus (SLE) can present with bilateral knee pain and swelling. These joint manifestations occur in up to 85 percent of SLE patients and often precede clinical diagnosis by months to years. The diagnosis is more likely in women in their 30s with bilateral pain. The first episode of gout often presents as a swollen, red, warm, and intensely painful knee, and acute flares may be accompanied by fever. Gout should be considered particularly in those with known risk factors who have impressive knee pain and an effusion with no clear inciting factor. The presentation of calcium pyrophosphate crystal deposition disease (ie, CPPD or “pseudogout”) may also closely mimic gout and in over 50 percent of cases the knee is the first joint affected. (See "Clinical manifestations and diagnosis of osteoarthritis" and "Arthritis and other musculoskeletal manifestations of systemic lupus erythematosus", section on 'Joint involvement' and "Clinical manifestations and diagnosis of gout" and "Clinical manifestations and diagnosis of calcium pyrophosphate crystal deposition (CPPD) disease".)

The presentation of some conditions that cause knee pain mimics that of acute joint infection. Such conditions, including prepatellar bursitis, may be due to overuse or develop following minor trauma that the patient has forgotten or ignored. Each of these conditions may cause alterations in joint fluid that mimic an occult infection. Sickle cell disease increases the risk for septic arthritis and osteomyelitis so these diagnoses should be investigated if such patients develop acute knee pain or swelling. (See "Knee bursitis" and "Septic arthritis in adults" and "Acute and chronic bone complications of sickle cell disease".)

Patients with hemophilia and other bleeding disorders can develop hemarthrosis from trauma or from normal activity when their coagulation status is not well controlled. Similarly patients with thrombocytopenia or those taking anticoagulant therapy may have similar knee pain and swelling. (See "Overview of hemarthrosis".)

Infrequently, acute nontraumatic knee pain may be caused by leukemia and other malignancies, Lyme disease, pigmented villonodular synovitis, or rheumatoid arthritis. Approximately 4 percent of adults and 14 percent of children with leukemic syndromes have arthralgia. Lyme disease often goes unrecognized and late manifestations may include acute knee pain. Rheumatoid arthritis presents in rare cases as a monoarticular arthritis before the full blown disease develops. (See "Malignancy and rheumatic disorders" and "Clinical manifestations of Lyme disease in adults" and "Clinical manifestations of rheumatoid arthritis".)

ADDITIONAL ULTRASOUND RESOURCES — Instructional videos demonstrating proper performance of the ultrasound examination of the knee and related pathology can be found at the website of the American Medical Society for Sports Medicine: anterior knee US examination, medial knee US examination, lateral knee US examination, posterior knee US examination, sports US knee pathology, US guided interventional procedures of the knee. Registration must be completed to access these videos, but no fee is required.

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: General issues in muscle and tendon injury diagnosis and management" and "Society guideline links: Muscle and tendon injuries of the lower extremity (excluding Achilles)" and "Society guideline links: Knee pain" and "Society guideline links: Patellofemoral pain" and "Society guideline links: Meniscal injury".)

SUMMARY AND RECOMMENDATIONS

●History – The history is critical to understanding the cause of musculoskeletal knee pain. Important elements include distinguishing between acute and chronic knee pain, identifying any trauma, learning the mechanism of injury if trauma was involved, and understanding what activities the patient participates in that may contribute to their symptoms. A review of useful questions to elicit the history is provided in the text. (See 'History' above.)

●Physical examination – The physical examination of any joint includes inspection, palpation, range of motion testing, strength and neurovascular testing, and special maneuvers to assess for specific diagnoses. Special tests are selected based upon the most likely diagnostic category, which is based in turn upon the history, including the mechanism of any injury and the chronicity (acute or chronic) of pain. The knee examination is described in detail separately. (See "Physical examination of the knee".)

●Three common diagnostic categories – Using information from the history, key symptoms, and findings from the basic knee examination, the clinician can usually select one of three common diagnostic categories that best fits the patient. The three major categories are:

•Acute knee pain, either from trauma or associated with overuse (table 1)

•Chronic knee pain associated with overuse (table 2 and table 3)

•Knee pain without trauma or overuse, possibly associated with systemic symptoms

●Acute pain associated with trauma – A history of acute knee pain associated with trauma is usually clear-cut, although the precise mechanism may be difficult to establish. Such a history allows the clinician to focus on the knee-related structures most likely to have been injured (table 1). These include the collateral and anterior cruciate ligaments and the menisci. Testing the stability of these structures, identifying points of focal tenderness and the presence of an effusion, and performing special tests to confirm suspicions about the structures involved comprise the core of the initial evaluation. (See 'Acute knee pain associated with trauma' above.)

●Chronic pain associated with overuse – Chronic knee pain associated with overuse has typically persisted for approximately six weeks or longer and is not associated with any sudden inciting trauma. Chronic knee pain associated with overuse is progressive, becoming more painful with increasingly less intense activity over time. Typically, the knee examination reveals no structural instability. Many such patients suffer from patellofemoral pain or another condition related to the patella or surrounding soft tissues (eg, pes anserinus, iliotibial band) (table 2). In older patients, a degenerative meniscus and osteoarthritis must be considered. (See 'Chronic knee pain associated with overuse' above.)

●Acute pain with activity but without trauma – Patients with knee pain that developed or increased abruptly after excessive activity but who clearly have not sustained trauma of any kind often suffer from acute-on-chronic pain related to overuse ("overuse trauma") (table 4). The pain associated with overuse trauma often arises towards the end of an activity that exceeds what the athlete has trained to do or follows some change in training. (See 'Acute knee pain associated with overuse' above.)

●Pain unrelated to trauma or overuse – Chronic or acute knee pain in an adult without inciting trauma or a history of overuse is a "red flag" that a more extensive evaluation is required, particularly if the pain is associated with constitutional symptoms, such as fever. The cause of such pain is much less likely to be musculoskeletal. (See 'Knee pain not associated with trauma or overuse' above.)